Method of forming a transistor gate of a recessed access device, method of forming a recessed transistor gate and a non-recessed transistor gate, and method of fabricating an integrated circuit

ABSTRACT

Methods and structures are provided for full silicidation of recessed silicon. Silicon is provided within a trench. A mixture of metals is provided over the silicon in which one of the metals diffuses more readily in silicon than silicon does in the metal, and another of the metals diffuses less readily in silicon than silicon does in the metal. An exemplary mixture includes 80% nickel and 20% cobalt. The silicon within the trench is allowed to fully silicide without void formation, despite a relatively high aspect ratio for the trench. Among other devices, recessed access devices (RADs) can be formed by the method for memory arrays.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 11/219,303, filed Sep. 1, 2005, entitled “SilicidedRecessed Silicon”, naming Hasan Nejad, Thomas A. Figura and Gordon A.Haller, Ravi Iyer, John Mark Meldrim, and Justin Harnish as inventors,the disclosure of which is incorporated by reference. This application,by claiming priority to U.S. patent application Ser. No. 11/219,303, isalso related to U.S. patent application Ser. No. 11/219,349, filed onSep. 1, 2005, entitled MEMORY CELL LAYOUT AND PROCESS FLOW and U.S.patent application Ser. No. 11/219,304, filed on Sep. 1, 2005, entitledPERIPHERAL GATE STACKS AND RECESSED ARRAY GATES.

FIELD OF THE INVENTION

This invention relates generally to silicidation reactions and theproducts thereof, and more particularly to the full silicidation ofsilicon in a recess.

BACKGROUND OF THE INVENTION

Integrated circuit designs are continually being scaled down in effortsto reduce power consumption and increase speed. With each passinggeneration, devices tend to get smaller and more densely packed, raisinga variety of problems for integration. One of the problems forintegration is the small volumes provided for conductive elements. Inorder to achieve acceptable circuit speeds, it is important that suchelements are provided with very high conductivity.

Other problems relate to difficulties in lining or filling high aspectratio trenches or vias. For example, elongated trenches are used fordamascene metallization; isolated holes or vias are used for formingvertical contacts; stacked trenches above the substrate and deeptrenches within the substrate are used for memory cell capacitorformation; etc. Depositing within such vias becomes more challengingwith higher aspect ratios with each passing generation. Voids can easilyform during deposition or subsequent processing, leading to lower deviceyields.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a method is provided forforming a metal silicide structure in an integrated circuit. The methodincludes providing a recess within a partially fabricated integratedcircuit. Silicon is deposited into the recess. A mixture of metals isdeposited over the recess and in contact with the silicon, where themixture of metals includes at least two metals having opposingdiffusivities relative to silicon. The mixture of metals and the siliconare reacted in the recess to form a metal silicide within the recess.

In accordance with another aspect of the invention, a method is providedfor forming a recessed access device for an integrated circuit. Themethod includes etching a trench in a semiconductor structure. Thetrench is lined with a dielectric layer, and the lined trench is atleast partially filled with silicon. A metal layer is deposited over thetrench and in contact with the silicon. The silicon in the trench isfully reacted in a silicidation reaction with the metal layer.

In accordance with another aspect of the invention, an integratedcircuit is provided, including a metal silicide structure. A metalsilicide fills at least a lower portion of a recess without voids. Themetal silicide includes a mixture of at least a first metal having agreater diffusivity in silicon than silicon has in the first metal. Themetal silicide also includes a second metal having a lesser diffusivityin silicon than silicon has in the second metal.

In accordance with another aspect of the invention, a memory device isprovided. The device includes a recessed access device in a memoryarray, including a recess within a semiconductor substrate, a thindielectric layer lining the recess, and a metal silicide filling atleast a lower portion of the trench without voids.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the detailed description ofthe preferred embodiments and from the appended drawings, which aremeant to illustrate and not to limit the invention.

FIG. 1 is a schematic plan view of a memory device, laid out inaccordance with a preferred embodiment of the invention.

FIG. 2 is a schematic, cross-sectional side view of the memory device ofFIG. 1 taken along lines 2-2, in accordance with a preferred embodimentof the invention.

FIGS. 3-7 are a series of cross-sectional views of a portion of asemiconductor device, illustrating formation of DRAM access transistorssimilar to those of FIGS. 1 and 2, according to a preferred embodimentof the present invention.

FIG. 8 is a schematic, cross-sectional view of the device of FIG. 7after recessing silicon within the trench, and prior to deposition ofmetal for silicidation, in accordance with one embodiment of the presentinvention.

FIG. 9 is a schematic, cross-sectional view of the device of FIG. 7after planarizing silicon within the trench and depositing metal forsilicidation, in accordance with another embodiment of the presentinvention.

FIGS. 10A-11B are micrographs illustrating fully silicided, recessedgates for memory access devices after a silicidation anneal is performedon the device of FIG. 9.

FIG. 12 is a schematic cross-section showing the partially fabricatedsemiconductor device of FIGS. 10A-11B after recessing and burying thefully silicided gates within their trenches.

FIGS. 13-21 are a series of cross-sectional views of a portion of asemiconductor device, illustrating simultaneous formation of peripheraltransistor gate stacks and recessed access devices (similar to those ofFIGS. 1 and 2) in the array, according to another embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the preferred embodiments of the present invention are illustratedin combination with a pitch doubling technique, it should be understoodthat the circuit design of these preferred embodiments may beincorporated into any integrated circuit. In particular, they may beadvantageously applied to form any device having an array of electricaldevices, including logic or gate arrays and volatile or non-volatilememory devices, such as DRAMs, RAMs, or flash memory. The integratedcircuits formed by the methods described herein can be incorporated inany of a number of larger systems, such as motherboards, desktop orlaptop computers, digital cameras, personal digital assistants, or anyof a number of devices for which memory is useful.

The design and functioning of one memory device, a DRAM, laid outaccording to one embodiment of the present invention, is illustrated inthe figures, and described in greater detail below.

FIG. 1 shows a view of a portion of a memory device 10. This schematiclayout illustrates the various electrical devices and other componentsthat form the memory device 10. Of course, many of these componentswould be indistinguishable in a purely visual representation, and someof the components shown in FIG. 1 are artificially distinguished fromother components in order to highlight their functionality. The memorydevice 10 is built on and in a substrate 11, which forms the lowestlevel of semiconductor material in which electrical devices are formed.The substrate 11 typically comprises silicon. Of course, other suitablematerials (e.g., other group III-V elements) may also be used, as iswell-known to those skilled in the art. When describing the othercomponents, their depth or height may be most easily understood withreference to the top surface of the substrate 11, best seen in FIG. 2.

Four elongate word lines 12 a, 12 b, 12 c, 12 d are also shown in FIG. 1extending along the memory device 10. In a preferred embodiment, theseword lines 12 were formed using a pitch doubling technique. Inparticular, these word lines 12 are preferably formed by a method thatwill be discussed in greater detail with reference to FIGS. 3-9. Usingsuch a technique, the pitch of the resulting features may be less thanthe minimum pitch defined by the photolithographic technique. Forexample, in one embodiment, the pitch of the resulting features mayequal one half the minimum pitch defined by the photolithographictechnique.

In general, pitch doubling may be performed by the following sequence ofsteps, as is well understood by those skilled in the art. First,photolithography may be used to form a pattern of lines in a photoresistlayer overlying a layer of an expendable material and a substrate. Thisphotolithographic technique achieves a pitch between adjacent lines of2F, as disclosed above, which pitch is limited by the opticalcharacteristics of photolithography. In one embodiment, F is within therange of 60 to 100 nm. This range is typical for state-of-the-artphotolithographic techniques used to define features. In onephotolithography system, F equals approximately 86 nm, while, in anothersystem, F equals approximately 78 nm.

The width of each line defined by photolithography is typically alsodefined as F, as would be well understood by those skilled in the art.The pattern may then be transferred by an etching step (preferablyanisotropic) to the lower layer of expendable material, thereby formingplaceholders, or mandrels in the lower layer. The photoresist lines canthen be stripped, and the mandrels can be isotropically etched toincrease the distance between neighboring mandrels. Preferably, thedistance between the neighboring mandrels is increased from F to 3F/2.Alternatively, the isotropic “shrink” or “trim” etch could have beenperformed at the level of the resist. A conformal layer of spacermaterial may then be deposited over the mandrels. This layer of materialcovers both horizontal and vertical surfaces of the mandrels. Spacers,i.e., material extending from sidewalls of another material, aretherefore formed on the sides of the mandrels by preferentially etchingthe spacer material from the horizontal surfaces in a directional spaceretch. The remaining mandrels are then selectively removed, leavingbehind only the spacers, which together may act as a mask forpatterning. Thus, where a given pitch, 2F, formerly included a patterndefining one feature and one space, the same width now includes twofeatures and two spaces defined by the spacers. As a result, thesmallest feature size achievable with a given photolithographictechnique is effectively decreased. This method of pitch doubling, whichmay be repeated for further reduction in the size of the features, willbe discussed in greater detail below with reference to FIGS. 3-9.

Of course, as would be well known in the art, the extent of theshrink/trim etch and the thicknesses of the deposited spacers may bevaried to achieve a variety of feature and pitch sizes. In theillustrated embodiments, whereas the photolithographic technique mayresolve a pitch of 2F, the features, i.e. word lines 12 in the instantexample, have a pitch of F. The word lines 12 are defined by a width ofabout F/2, and adjacent word lines 12 a, 12 b or 12 c, 12 d areseparated by the same width, F/2. Meanwhile, as a byproduct of thepitch-doubling technique, the separation between the spaced-apart wordlines 12 b, 12 c is 3F/2. In a preferred embodiment, an isolation trenchis filled with an insulator and lies within this separation betweenthese word lines 12 b, 12 c; however, in other embodiments, thisisolation trench need not be present.

For every distance of 3F, there are two word lines, yielding what may bereferred to as an effective pitch of 3F/2. More generally, the wordlines preferably have an effective pitch between 1.25F and 1.9F. Ofcourse, the particular pitch used to define the word lines is only anexample. In other embodiments, the word lines may be fabricated by moreconventional techniques, and pitch doubling need not be used. In oneembodiment, for example, the word lines may each have a width of F andmay be separated by F, 2F, 3F or some other width. In still otherembodiments, the word lines need not be formed in pairs either. Forexample, in one embodiment, only one word line need pass through eachactive area.

The entire length of the word lines 12 is not visible in FIG. 1, but, ina typical implementation, each word line 12 may extend across hundreds,thousands or millions of transistors. At the edges of the word lines 12,as is well-known to those of skill in the art, the word lines 12 aretypically electrically coupled to a device, such as a power source, thatcan place a current across the word line 12. Often, the power sourcesfor the word lines 12 are indirectly coupled to a CPU through a memorycontroller.

In one embodiment, the word lines 12 comprise a p-type semiconductor,such as silicon doped with boron. In other embodiments, the word lines12 may comprise an n-type semiconductor, metal silicide, tungsten orother similarly behaving material, as is well-known to those of skill inthe art. In some embodiments, the word lines 12 may comprise a varietyof materials, in a layered, mixed or chemically bonded configuration.

The horizontal lines seen in FIG. 1 are formed by digit lines 14 a, 14b. In one exemplary embodiment, the width of each of these digit lines,illustrated as DL in FIG. 1, is equal to F. No pitch doubling has beenused to form these exemplary digit lines 14. Adjacent digit lines 14 a,14 b are separated, in a preferred embodiment, by a distance,illustrated as S in FIG. 1, equal to 2F. The pitch of the digit lines ispreferably greater than 2.5F, and preferably less than 4F. Withoutpitch-doubling techniques, the lower limit is, of course, imposed by thephotolithographic technique used to form the digit lines. On the otherhand, near the upper end of this range, the photolithography is lessprecise, and therefore less expensive, but the memory itself begins togrow too large. In a more preferred embodiment, the pitch of the digitlines is between 2.75F and 3.25F. This range represents a desirablebalance between the ease of manufacturing and the size of the chip. Inthe illustrated embodiment, the digit lines 14 have a pitch of 3F. Ofcourse, in other embodiments, different widths and spacing are possible.

As with the word lines 12, the entire length of the digit lines 14 isalso not visible in FIG. 1, and the digit lines 14 typically extendacross many transistors. At the edges of the digit lines 14, as iswell-known to those of skill in the art, the digit lines 14 aretypically electrically coupled to current sense amplifiers, and therebyto a power or voltage source. Often, the power sources for the digitlines 14 are also indirectly coupled to a CPU through a memorycontroller. As a result of the more relaxed pitch between the digitlines 14, the sense amplifiers may be spaced farther from one another,relaxing their manufacturing tolerances, and decreasing the likelihoodof capacitance coupling of adjacent digit signals.

In one embodiment, the digit lines 14 comprise a conducting metal, suchas tungsten, copper, or silver. In other embodiments, other conductorsor semiconductors may be used, as is well-known to those of skill in theart.

The other features visible in FIG. 1 are the active areas 16,illustrated within curvilinear rectangles, which form axes A that areangled relative to the axes B of the digit lines. These rectanglesrepresent a doped region or well within the substrate 11; however, inother embodiments, these rectangles need not represent physicalstructures or materials within or upon the memory device 10 andsubstrate 11. The active areas 16 define those portions of the memorydevice 10 that contain field effect transistors and are typicallysurrounded by field isolation elements (e.g., shallow trench isolation(STI)). In one preferred embodiment, these active areas each comprisetwo drains 18 and one source 20. The source and drains may be larger orsmaller than illustrated in FIG. 1, as is well known to those of skillin the art. They may also be fabricated in any of a number of wayswell-known to those of skill in the art.

In another embodiment, the active areas may comprise one source and onedrain, wherein the source is formed near the digit line, and the drainis separated from the source by a word line. In such an embodiment, thememory device may be configured similarly to the memory device 10 inFIG. 1, but there need only be one word line passing through each activearea. Of course, in another embodiment, an active area may comprise onesource and one drain, and the memory device may further comprise twoword lines extending near the active area, configured similarly to thepaired word lines 12 c, 12 d shown in FIG. 1. In such an embodiment, thetwo word lines may both extend between the source and drain, and provideredundant control of the transistor.

As illustrated, a digit line 14 runs proximal to, and preferably above(see FIG. 2), each source 20 that lies in the digit line's row.Meanwhile, each source 20 is separated to either side from its adjacentdrains 18 by word lines 12. In one embodiment, the source 20 and drains18 comprise an n-type semiconducting material, such as silicon dopedwith phosphorous or antimony. In other embodiments, the source 20 anddrains 18 may comprise a p-type semiconductor, or they may be fabricatedfrom other materials, as is well-known to those of skill in the art. Infact, the source 20 and drains 18 need not be fabricated from the samecompounds.

The functioning of memory device 10 is briefly discussed with referenceto FIG. 2, which shows a cross-sectional view of one of the active areas16. For a further discussion of the basic manner in which DRAMsfunction, U.S. Pat. No. 3,731,287, issued to Seely et al., which isincorporated by reference herein in its entirety, discusses DRAMs ingreater detail.

As shown in FIG. 2, the drains 18 and source 20 may comprise protrusionsfrom the relatively flat, upper surface of the substrate 11. In onepreferred embodiment, the source 20 and drains 18 are fabricated asone-piece with the substrate 11, and are raised relative to the surfaceof the substrate 11 by etching a monolithic wafer or substrate; inanother arrangement, the source and drain protrusions are formed byselective epitaxial deposition using techniques well-known to those ofskill in the art.

In one embodiment, at least a portion of digit line 14 b is locatedabove the upper surface of source 20. As illustrated in FIG. 2, thesource 20 is electrically coupled to the digit line 14 b by a digit lineplug 22, which plug may be formed in multiple stages or in a singlestage, as shown. Meanwhile, the source 20 is separated from the twodrains 18 by word lines 12 a, 12 b. The word lines 12 a, 12 b arepreferably embedded in the substrate 11, extending downwards from thesurface. Transistors of this design are often referred to as recessedaccess devices or RADs. The drains 18 are, in turn, electrically coupledto storage capacitors 24, and, in particular, to the lower electrode 26of the storage capacitors 24, by contact plugs 28. In a preferredembodiment, the storage capacitors 24 comprise a lower electrode 26separated from a reference electrode 30 by a dielectric material 32. Inthis configuration, these stacked storage capacitors 24 function in amanner well known to those of skill in the art. As illustrated, thestorage capacitors 24 are preferably located above the plane of thesubstrate 11, although trench capacitors can be used in otherarrangements.

In one embodiment, one side of every storage capacitor 24 forms areference electrode 30, while the lower electrode 26 is electricallycoupled to an associated drain 18. The word lines 12 a, 12 b function asgates in the field effect transistors they pass through, while the digitline 14 b functions as a signal for the sources to which it iselectrically coupled. Thus, the word lines 12 a, 12 b preferablycontrol-access to the storage capacitors 24 coupled to each drain 18, byallowing or preventing the signal (representing a logic “0” or a logic“1”) carried on the digit line 14 b to be written to or read from thestorage capacitors 24. Thus, each of the two capacitors 24 connected toan associated drain 18 can contain one bit of data (i.e., a logic “0” orlogic “1”). In a memory array, the combination of the digit line andword line that are selected can uniquely identify the storage capacitor21 to or from which data should be written or read.

Turning back then to FIG. 1, the design and geometry of the memorydevice 10 may be discussed in further detail. In the lower right handcorner of FIG. 1, a number of axes have been illustrated. These axes aregenerally aligned with the longitudinal axes of circuit elements formingthe memory device 10, and are illustrated to more clearly show theangles formed between various electrical devices and components. Axis Arepresents the longitudinal axis of active area 16. The drains 18 andsource 20 of each active area 16 preferably have a substantially linearrelationship that may be used to define a longitudinal axis. Asillustrated, all of the active areas 16 are substantially parallel. Itwill be understood, of course, that the drains 18 and source 20 need notform an absolutely straight line, and indeed a substantial angle may bedefined by these three points. In some embodiments, therefore, the axisA may be defined by the two drains 18, or by the source 20 and only oneof the drains 18, or in a number of other ways that would be clearlyunderstood by those skilled in the art. In other embodiments, in whichthe active area comprises a single drain and a single source, the axis Amay be defined by a line between the single drain and single source.

Axis B represents the longitudinal axis of digit line 14 b. In theillustrated embodiment, the digit line 14 b forms a substantiallystraight line. Just as the active areas 16 are preferably parallel, thedigit lines 14 a, 14 b also preferably form generally parallel axes.Thus, in a preferred embodiment, axis A of every active area 16 forms asimilar angle with every axis B of the digit lines 14, at least in theregion of each memory cell.

In a preferred embodiment, illustrated in FIG. 1, an acute angle isformed between axis A and axis B. In the illustrated embodiment, thisacute angle, θ, defined between axis A and axis B, is 45°.

The angling of the active areas 16 relative to the digit lines 14facilitates the location of the contact plugs 28 extending betweendrains 18 and associated storage capacitors 24. Since these contactplugs 28 extend from the top surface of the drains 18 in the preferredembodiment (illustrated in FIG. 2), the engineering is simplified if thedigit lines 14 do not extend over the tops of the drains 18. By anglingthe active areas 16, the distance between a digit line 14 and drains 18may be selected to facilitate electronic contact between the drains andcontact plugs, even while the digit line 14 substantially overlaps andcontacts the source 20 of the same active area 16.

Of course, the angle, θ, may have any of a number of values chosen tomaximize the pitch of the electrical devices. As will be readilyapparent to one of skill in the art, different angles will yielddifferent pitches between adjacent active areas. In one embodiment, theangle, θ, is preferably between 10° and 80° degrees. In a more preferredembodiment, the angle, θ, is between 20° and 60°. In a still morepreferred embodiment, the angle, θ, is between 40° and 50°.

Turning to FIGS. 3-10, one method of fabricating the pitch-doubled wordlines 12 of the memory device 10 is illustrated in greater detail. Theskilled artisan will readily appreciate that the particular materials ofthe illustrated embodiment can be replaced individually or incombination with other groups of materials. FIG. 3 illustrates asemiconductor substrate 11 over which a thin, temporary layer 40,comprising oxide in a preferred embodiment, has been formed according toconventional semiconductor processing techniques. A hard mask layer 42,such as silicon nitride, is then deposited over the substrate 11 andtemporary layer 40. The hard mask layer 42 may be formed by anywell-known deposition process, such as sputtering, chemical vapordeposition (CVD) or low-temperature deposition, among others. Althoughthe hard mask layer 42 comprises silicon nitride in the preferredembodiment, it must be understood that it may also be formed of siliconoxide, for example, or other materials suitable for the selective etchsteps described below.

Next, in a step not illustrated in the figures, the hard mask layer 42is patterned using a photoresist layer formed over the hard mask layer42. The photoresist layer may be patterned to form a mask usingconventional photolithographic techniques, and the hard mask layer 42may then be anisotropically etched through the patterned photoresist toobtain a plurality of hard mask columns 44 extending in the y-dimension(as defined by FIG. 1), with trenches 46 separating those columns. Thephotoresist layer may then be removed by conventional techniques, suchas by using an oxygen-based plasma.

With reference to FIG. 5A, after the trenches 46 have been formed in thehard mask layer 42, a conformal layer of spacer material may bedeposited to cover the entire surface of the memory device 10.Preferably, the spacer material can be selectively etched with respectto the substrate 11 and the temporary layer 40, and the substrate 11 andthe temporary layer 40 can each be selectively etched with respect tothe spacer material. In the illustrated embodiment, the spacer materialcomprises polysilicon. The spacer material may be deposited using anysuitable deposition process, such as, for example, CVD or physical vapordeposition (PVD).

After laying the spacer material over the vertical and horizontalsurfaces of the memory device 10, an anisotropic etch may be used topreferentially remove the spacer material from the horizontal surfacesin a directional spacer etch. Thus, the spacer material is formed intospacers 48, i.e., material extending from the sidewalls of anothermaterial. As shown in FIG. 5, spacers 48 are formed within the trench 46and narrow it.

With reference to FIG. 5B, a second hard mask layer 49 may then bedeposited over the entire surface of the memory device 10. This layer ofhard mask 49, also silicon nitride in a preferred embodiment, ispreferably deposited to a thickness sufficient to fill the trench 46. Ofcourse, the hard mask material 49 may be deposited by any of a number ofsuitable deposition processes, including CVD or PVD. After deposition ofa sufficient amount of hard mask material 49, the excess that may haveformed over the spacers 48 and over the other portions of previouslydeposited hard mask 42 may be removed by any of a number of processeswell-known to those of skill in the art. For example, the surface of thedevice 10 may be planarized to the level of the dotted line of FIG. 5B,such that the sidewalls of the remaining spacers 48 are nearly vertical.Any suitable planarization process, such as, for example, chemicalmechanical planarization may be used.

The spacers 48 that are now exposed at the top surface of the memorydevice 10 may be stripped using any of a number of processes. In theillustrated embodiment, a process may be used that selectively stripspolysilicon relative to silicon nitride. For example, in one embodiment,a selective wet etch may be used. The trenches formed where the spacers48 have been etched are further deepened by a secondary etch thatselectively etches the temporary layer 40 as well as the substrate 11.These trenches are also preferably formed using a directional process,such as, for example, ion milling or reactive ion etching.

FIG. 6 illustrates the result of these processes, with openings orrecesses in the form of trenches 50 separated by less than the minimumpitch possible using photolithographic techniques alone. Preferably thetrenches 50 have a width at top between about 25 nm and 75 nm. Ofcourse, a skilled artisan will appreciate that numerous other techniquesfor pitch multiplication may be used to arrive at the stage shown inFIG. 6. Many such techniques will generally include a spacer process, bywhich physical deposition can achieve a smaller pitch thanphotolithographic techniques alone. The trenches 50 typically also havean aspect ratio greater than 1:1, and preferably greater than 2:1.Increased depth maximizes available volume and thence conductivity forthe word lines, at the expense of difficulty in filling with a suitablematerial.

After formation of these trenches 50, the hard mask layer 42 isselectively stripped, by any of a number of methods well known to thoseof skill in the art. In FIG. 7, a gate dielectric layer 54 is blanketdeposited or thermally grown over the device, lining the inner surfacesof the trenches 50. The illustrated gate dielectric layer 54 comprisessilicon oxide formed by thermal oxidation in a preferred embodiment, butcan also be a deposited high K material in other embodiments. A layer ofgate material 52, which comprises polysilicon in the illustratedembodiment, may then also be blanket deposited over the entire memorydevice 10. In one embodiment, the gate layer 52 completely fills thetrenches 50 and forms a top surface of the device 10. In a preferredembodiment, this polysilicon is undoped.

After a series of doping steps to define the drains and sources oftransistors, the undoped polysilicon in the trenches 50 is etched backuntil the top of the gate layer 52 resides beneath the top surface ofthe substrate 11. This stage of the process is shown in FIG. 8. Therecessed polysilicon 52 of FIG. 8 can serve as the word lines and thegate electrodes for the memory cell transistors if appropriately doped.

Preferably, however, the gate electrodes in the arrays are formed of amore highly conductive material than traditional polysilicon gates. Thisis due to the fact that the recessed gates 12 (see FIGS. 1 and 2) aremore narrow than the typical gate electrode. Metallic materialscompensate, in whole or in part, for the small volume of the gates inthe array, improving lateral signal propagation speed along the wordlines. Thus, the undoped polysilicon of FIG. 8 can be silicided afterrecessing by depositing metal thereover and reacting. Metal silicide canhave better than 10 times the conductivity of doped polysilicon anddemonstrate a suitable work function.

With reference to FIGS. 9-12, in another arrangement, rather than beingrecessed, the polysilicon 52 is initially etched back or planarized downto the gate oxide 54, thus isolating the polysilicon within the trenches50 without recessing at this stage. The polysilicon of the gate layer 52within the trenches 50 is subjected to a salicidation (self-alignedsilicidation) reaction to form a layer of conductive material 56. Ametal layer 55 (FIG. 9) may be blanket deposited and an anneal step mayform a silicide material 56 (FIG. 12) wherever the metal contactssilicon, such as over the polysilicon gate layers 52.

In one embodiment, the silicided material comprises silicon and one ormore metals, such as, for example, tungsten, titanium, ruthenium,tantalum, cobalt or nickel. A selective metal etch removes the excessmetal but does not remove the silicide 56. The metal silicide 56 therebyforms a self-aligned layer that increases the lateral conductivity alongthe word line.

Preferably, the gate layer 52 is fully silicided to maximize lateralconductivity. Full reaction also assures silicide formation down to thebottom of the trenches 50. In the illustrated recessed access devices(RADs), the channel extends across not only the bottom of the gate, butalso along the gate's sidewalls. Accordingly, incomplete silicidationwould result in different work functions along the length of the RADchannel. Furthermore, full silicidation ensures similar gate workfunctions across the array, from array to array across a wafer, and fromwafer to wafer. It has been found difficult, however, to achieve fullsilicidation within the tight confines of the illustrated trenches 50,with a single metal to form the conductive material 56. Either nickel orcobalt, for example, tends to form voids in the high-aspect ratiotrenches 50. Other metals have demonstrated similar difficulties forfull silicidation for recessed access devices. The skilled artisan willappreciate that full silicidation can be challenging for material withinother types of recesses, such as contact openings or vias, stackedcontainer shapes for capacitors, capacitor trenches, etc.

Without wanting to be bound by theory, the voiding appears to be causedby diffusion during the silicidation reaction, in combination with thetight confines of the high aspect ratio trenches 50. Silicon diffusesmore readily in cobalt than cobalt does into silicon. Accordingly,silicon tends to migrate during the reaction, leaving voids in thetrenches 50. Furthermore, a high temperature phase transformation annealto convert the silicide from CoSi to the more stable CoSi₂. Nickel, onthe other hand, diffuses more readily into silicon than silicon doesinto nickel and so also has a tendency to create voids during thereaction in which NiSi is converted into the NiSi₂ phase.

Accordingly, the metal layer 55 preferably comprises a mixture ofmetals, where at least two of the metals in the mixture have opposingdiffusivities relative to silicon. For example, the metal layer 55 cancomprise a mixture of nickel and cobalt, such that the directions ofdiffusion tend to balance each other and minimize the risk of voiding.In this example, the cobalt preferably comprises less than 50 at. % ofthe mixed metal 55, and more preferably the mixture comprises about70-90 at. % Ni and about 10-30 at. % Co. Such a mixture of nickel andcobalt has been found to more readily accomplish full silicidation ofthe gate layer without voiding, thus increasing signal propagationspeeds along the word line. In contrast to partial silicidation, fullysilicided word lines are not only more conductive, but also will ensureconsistent work function along the length of the channel. Fullsilicidation will also demonstrate better consistency from device todevice across an array, from array to array, or wafer to wafer, sincepartial silicidation will tend to leave inconsistent compositionsdepending upon local temperature variations, etc.

In one example, a sputtering target comprising 80% Ni and 20% Co issputtered over the polysilicon 52 to produce the metal layer 55. Thesubstrate is then subjected to a silicidation anneal. While a hightemperature (e.g., 800° C.) anneal is possible for a shorter time,preferably the anneal is conducted at lower temperatures for a longertime. For example, the substrate is annealed at 400-600° C. for 25-35minutes. In experiments, the silicidation anneal was conducted in abatch furnace under an N₂ environment at 500° C. for 30 minutes.

In view of the disclosure herein, the skilled artisan can readily selectother suitable mixtures of metals for full silicidation within trenches.Examples of metals that diffuse more readily in silicon than silicondoes in that metal include Ni, Pt and Cu. Examples of metals in whichsilicon diffuses more readily than the metal diffuses in silicon includeCo, Ti and Ta.

FIGS. 10A-11B are micrographs showing recessed, fully silicidedNi_(x)Co_(y)Si_(z) gate material within 50 nm wide trenches lined withsilicon oxide. FIGS. 10A and 10B show cross sections across the width oftwin trenches, at two different magnifications. FIGS. 11A and 11B showcross sections along the length of one of the trenches, at two differentmagnifications. The trenches have a width at the top of about 50 nm anda depth of about 150 nm, such that the aspect ratio of these trencheswas about 3:1. A smooth, uniform composition is observed, filling atleast a lower portion of the trenches without voiding. In the example ofFIGS. 11-12, after depositing the polysilicon 52 (FIG. 7), thepolysilicon can be etched back only to the gate dielectric top surface54, thus isolating the silicon within the trenches without recessing.

Referring now to FIG. 12, the silicided layers 56 can be recessed withinthe trenches and are then covered by a second insulating layer 58, suchas silicon nitride. These insulating layers 58 may be deposited and thenetched or planarized. The conductive material 56 thereby forms the wordlines 12 a, 12 b of the completed memory device 10, and the word lines12 a, 12 b are separated from the other circuit elements by theinsulating layers 58. Thus, as would be well understood by those ofskill in the art, the word lines 12 have been pitch-multiplied, and havea pitch roughly one half of that possible simply using photolithographictechniques. Note, however, that certain aspects of the disclosure hereinprovide advantages whether or not the word lines are pitch-multiplied.

Of course, in other embodiments, the pitch-multiplication may take placeby any of a variety of processes well-known to those skilled in the art.

The silicided layers 56 of the illustrated embodiment thus fill lowerportions of the trenches 50, preferably filling greater than 50% of thetrench heights, more preferably filling greater than 75% of the trenchheight. In the illustrated embodiment, about 70-90 at % of metal in themetal silicide 56 is nickel and about 10-30 at % of metal in the metalsilicide is cobalt.

As will be appreciated by the skilled artisan, in a preferredembodiment, the logic in the periphery is preferably simultaneouslydefined as certain of the above steps are completed, thereby making thechip-making process more efficient. In particular, the silicon and metaldeposition steps to define recessed word lines preferably simultaneouslydefine gate electrodes over the substrate for the CMOS transistors inthe periphery.

Referring to FIGS. 13-21, in accordance with another embodiment,different work functions and resistivity can be established for thesimultaneously processed gate electrodes in the array and the logicregions in the periphery. In the illustrated embodiment, this isfacilitated by etching array RAD trenches through a polysilicon layer,which forms part of the gate stack in the periphery.

With reference to FIG. 13, a polysilicon layer 60 can be deposited overthe substrate 11 prior to forming the trenches. The polysilicon layer 60can be first deposited over a thin dielectric 54 a (e.g., grown gateoxide). The substrate can then be patterned with a pitch-doubled mask(not shown), such as that described with respect to FIGS. 3-6. An etchstop layer 61 is also formed, in the illustrated embodiment comprisingabout 100-200 Å of TEOS-deposited oxide.

With reference to FIG. 14, the trenches 50 are etched through theoverlying etch stop layer 61, the polysilicon layer 60, the underlyingdielectric 54 a and the substrate 11. The gate dielectric 54 b can thenbe formed over the exposed portions of the substrate 11, such as byoxidation of the trench walls. Due to the pre-existing etch stop layer61, no significant further oxide grows over the top surface of thepolysilicon 60, as shown.

Subsequently, as shown in FIG. 15, a metallic material 62 can bedeposited over the polysilicon 60 and into the trenches 50. As describedwith respect to FIGS. 9-12, the trenches 50 are preferably filled withmaterial more conductive than polysilicon. In the illustratedembodiment, the metallic material 62 comprises titanium nitride (TiN).

With reference to FIG. 16, the metallic material 62 is preferably etchedback or planarized to leave isolated lines of the conductive material 62in the trenches 50, stopping on the oxide etch stop layer 61 (see FIG.15). Following etch back, the etch stop layer 61 overlying thepolysilicon layer 60 is removed (e.g., using an HF dip for the preferredoxide material of the etch stop layer 61), while the dielectric layer 54b within the trenches 50 is protected by the metallic material 62.Subsequently, metallic layers 64, 66 are deposited over the siliconlayer 60. As will be appreciated by the skilled artisan, the firstdielectric layer 54 a, the polysilicon layer 60, and the overlyingmetallic layers 64, 66 can serve as the transistor gate stack in theperiphery. All these layers are deposited in both regions of interest(in the memory example, in both periphery and memory array regions).Polysilicon can be variably doped to establish a desired transistor workfunction, such that a single material deposition, and different dopingsteps, can be used to define gates for both NMOS and PMOS of a CMOScircuit. The overlying metallic layer 66 can serve to improve lateralsignal propagation speeds along lines controlling the gates, andcomprises tungsten (W) in the illustrated embodiment. The interveningmetallic layer 64 can ensure physical and electrical compatibility(e.g., fulfilling adhesion and barrier functions) at the juncturebetween the polysilicon layer 60 and the overlying metallic layer 66,and in the illustrated embodiment comprises titanium nitride, and moreparticularly metal-rich metal nitride.

Referring to FIG. 17, the gate stack also includes a cap layer 68,formed of silicon nitride in the illustrated embodiment. FIG. 17 showsthe trenches 50, filled with the metallic material 62, in a first ormemory array region 70 of the substrate. The gate stacks layers 54 a,60, 64, 66 and 68 extend across both the array region 70 and the secondor periphery or logic region 72 of the substrate. A photoresist mask 76is configured for patterning transistor gates in the periphery 72.

As shown in FIG. 18, a series of etch steps etches first through the caplayer 68, including a metal etch to remove the metallic layer(s) 64, 66.Chlorine-based reactive ion etch (RIE), for example, can selectivelyremove typical metallic materials, such as the illustrated tungstenstrapping layer 66 and intervening metal nitride layer 64, whilestopping on the underlying polysilicon layer 60. A high degree ofselectivity enables continuing the metal etch after exposure of thepolysilicon 60 until the metallic material 62 is recessed in thetrenches 50, as shown.

Referring now to FIG. 19, the etch chemistry can be switched followingrecessing of the metallic gate material 62 in the array trenches, andthe silicon 60 can be patterned using the same mask 76, completingpatterning of the gate stacks 80 for the periphery 72.

Referring now to FIG. 20, following removals of the mask, a spacer layer84 is deposited over the substrate, coating the gate stacks 80conformally but filling the recesses at the top of the array trenches50. In the illustrated embodiment, the spacer layer 84 comprises siliconnitride, but the skilled artisan will appreciate that a number ofdifferent insulating materials can be used.

As shown in FIG. 21, a subsequent spacer etch (directional etch) leavessidewall spacers 86 along sidewalls of the gate stacks 80, allowingself-aligned doping of source/drain areas. In the array 72, however,because the shallow recesses at the top of the trenches are filled withthe spacer layer 84 (see FIG. 20), the spacer etch merely etches thespacer material back in the array 72, leaving an insulating cap layer 88burying the gate material 62 within the trenches 50.

The skilled artisan will appreciate that various doping steps for CMOStransistors, including source/drain, channel enhancement, gateelectrode, lightly doped drain (LDD) and halo doping, are omitted in thedescription herein for simplicity.

The embodiment of FIGS. 13-21 thus facilitates simultaneous processingof transistors in the array and the periphery. In the illustratedembodiment, the array transistors are recessed access devices (RADs),whereas the peripheral gates are formed above the substrate 11 asconventional planar MOS transistors. While described in the context ofconventional CMOS circuitry in the periphery, the skilled artisan willappreciate that the peripheral transistors can take other forms.Advantageously, in the illustrated embodiment, the metallic layer in theRAD trenches can be recessed at the same time as patterning theperipheral gate stacks. Furthermore, the peripheral sidewall spacers aresimultaneously formed with the insulating cap on the RAD gates or wordlines.

Although not shown, it will be understood that conventional DRAMfabrication techniques may be used to create the other circuit elementsshown in FIG. 2. For example, different levels of doping may be used toform the drains 18 and source 20 of FIG. 2, and the stacked storagecapacitors 24 may be formed according to a plurality of deposition andmasking steps.

As a result of the device layout and its method of manufacture, thecompleted memory device 10 shown in FIGS. 1 and 2 possesses a number ofadvantages in comparison to conventional DRAM. For example, the size ofeach memory cell and the overall size of the memory device 10 may besubstantially reduced without a corresponding, substantial reduction inthe distance between adjacent sense amplifiers. Moreover, the word lines12 and digit lines 14 may have substantially different pitches, whichenables the digit lines 14 to have far greater separation than the wordlines 12. For example, in the preferred embodiment, the word lines 12have an effective pitch of 1.5F, while the digit lines 14 may have apitch of 3F. In addition, the steps for forming the digit lines 14 andword lines 12 are simplified by making them substantially linear andgenerally perpendicular to one another, while realizing space-savings byplacing the active areas 16 at an angle to these elements. The wordlines 12 in the preferred embodiment are also recessed, and, unlike thelayout in conventional DRAM, there is no spacer using up valuable spacebetween the gates and the sources or drains of the active areas (as maybe easily seen in FIG. 2). Thus, the memory device 10 may be made moredense.

Furthermore, the use of a mixture of metals facilitates fullsilicidation of the silicon buried within trenches 50 without theharmful formation of voids. Accordingly, a high conductivity can beachieved for the relatively small volume word lines.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the invention. Indeed, the novel methodsand devices described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and devices described herein may be made withoutdeparting from the spirit of the invention. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the invention.

1. A method of forming a gate of a recessed access device, comprising:forming a trench within semiconductive material of a substrate, thetrench having a top width that is less than 60 nm; depositing gatematerial comprising polysilicon to within the trench; depositing a metallayer over the semiconductive material, over the polysilicon and withinthe trench, the metal layer comprising at least two different metals,the two different metals having opposing diffusivities relative tosilicon; subjecting the polysilicon and the metal layer to asilicidation anneal to form metal silicide within the trench; andforming an insulator over the metal silicide.
 2. The method of claim 1wherein the silicidation anneal transforms all of the polysilicon tometal silicide.
 3. The method of claim 1 wherein the silicidation annealforms the metal silicide at the bottom of the trench.
 4. The method ofclaim 1 wherein the two different metals comprise nickel and cobalt. 5.The method of claim 4 wherein the metal layer has less cobalt thannickel.
 6. The method of claim 1 wherein one of the two different metalscomprises at least one of Ni, Pt, and Cu, and another of the twodifferent metals comprises at least one of Co, Ti, and Ta.
 7. The methodof claim 6 wherein the two different metals comprise nickel andtitanium.
 8. The method of claim 6 wherein the two different metalscomprise nickel and tantalum.
 9. The method of claim 6 wherein the twodifferent metals comprise platinum and cobalt.
 10. The method of claim 6wherein the two different metals comprise platinum and titanium.
 11. Themethod of claim 6 wherein the two different metals comprise platinum andtantalum.
 12. The method of claim 6 wherein the two different metalscomprise copper and cobalt.
 13. The method of claim 6 wherein the twodifferent metals comprise copper and titanium.
 14. The method of claim 6wherein the two different metals comprise copper and tantalum.
 15. Themethod of claim 1 comprising recessing the metal silicide within thetrench.
 16. The method of claim 15 comprising after recessing the metalsilicide within the trench, forming the insulator within the trench overthe metal silicide.